Interruption of power on present day AC electrical distribution systems can present problems for AC electric motors driving high inertia loads. Under some conditions the dropout of the main electroresponsive connecting and disconnecting contactors may be prevented from occurring rapidly enough to prevent development of large surge currents that will be imposed on the motor when power is reestablished. Such surge currents can cause large torque shocks on the motor and driven machinery.
The main motor supply contactors have their coil energizing control circuits supplied from step-down control transformers that have their primary windings connected for supply ahead of the contactor power contacts. However, when power supply interruptions occur, energizing current to a control transformer is not immediately cut-off, due to the sustaining effect of the feedback motor counter voltage. Thus, the energizing coil of the main motor supply contactor can be held sufficiently energized for a period preventing rapid dropout of the contactor. If the power supply outage is in a critical duration range the aforementioned surge currents can occur due to electrical phase slippage between the motor and supply voltages. With motors driving high inertia loads, such critical duration ranges can readily occur because motor counter voltage will be maintained at high levels for longer periods when power interruptions occur.
To prevent such large surge currents, and possible attendant damage to motors and driven machinery from occurring it is desirable that the energizing circuit connections to the operating coils of such contactors be deliberately opened if the power outage to the motor persists more than a few cycles of the power supply frequency.